Technical Field
The present disclosure relates to an electroluminescent display device, and more particularly, to an electroluminescent display device capable of improving light efficiency and color properties.
Description of the Related Art
Recently, flat panel displays have been widely developed and applied to various fields because of their thin profile, light weight, and low power consumption.
Among the flat panel displays, electroluminescent display devices emit light due to the radiative recombination of an exciton after forming the exciton from an electron and a hole by injecting charges into a light emitting layer between a cathode for injecting electrons and an anode for injecting holes.
The electroluminescent display devices include a flexible substrate such as plastic; because they are self-luminous, the electroluminescent display devices have excellent contrast ratios; the electroluminescent display devices have a response time of several micro seconds, and there are advantages in displaying moving images; the electroluminescent display devices have wide viewing angles and are stable under low temperatures; since the electroluminescent display devices are driven by a low voltage of direct current (DC) 5V to 15V, it is easy to design and manufacture driving circuits; and the manufacturing processes of the electroluminescent display device are simple since only deposition and encapsulation steps are required.
FIG. 1 is a view of illustrating a band diagram of a related art electroluminescent display device.
In FIG. 1, the electroluminescent display device includes an anode 1, a cathode 7 and a light emitting material layer 4 between the anode 1 and the cathode 7. A hole transporting layer (HTL) 3 is disposed between the anode 1 and the light emitting material layer 4 for injecting holes into the light emitting material layer 4, and an electron transporting layer (ETL) 5 is disposed between the cathode 7 and the light emitting material layer 4 for injecting electrodes into the light emitting material layer 4. At this time, to further efficiently inject the holes and the electrons, a hole injecting layer (HIL) 2 can be disposed between the anode 1 and the hole transporting layer 3, and an electron injecting layer (EIL) 6 can be disposed between the cathode 7 and the electron transporting layer 5.
In the electroluminescent display device having the above-mentioned structure, a hole (+) injected into the light emitting material layer 4 through the hole injecting layer 2 and the hole transporting layer 3 from the anode 1 is combined with an electron (−) injected into the light emitting material layer 4 through the electron injecting layer 6 and the electron transporting layer 5 from the cathode 7, whereby an exciton 8 is generated and light is emitted from the exciton 8. Here, the light has a color corresponding to a band gap of the light emitting material layer 4.
The electroluminescent display devices can be classified into a bottom emission type, a top emission type and a both side emission type according to an output direction of light emitted form the light emitting material layer 4. The light emitted from the light emitting material layer 4 can be outputted to the outside through the anode 1 in the bottom emission type, can be outputted to the outside through the cathode 7 in the top emission type, and can be outputted to the outside through both the anode 1 and the cathode 7.
In the electroluminescent display devices, the cathode 7 can be formed of a metallic material. The metallic material of the cathode can become diffused into the light emitting material layer 4, thereby lowering characteristics of a light emitting diode.
Namely, since the metallic material can become diffused into the light emitting material layer 4, light efficiency and color properties decrease. In addition, a driving voltage of the light emitting diode increases.
These problems can occur more severely in the top emission type and the both side emission type in which the cathode 1 is transparent.